1. Field of the Invention
This invention relates to a method to prepare active silicon/copper contact mass for the direct reaction of organic halides with silicon to make organohalosilanes. More particularly, this invention utilizes vapor of a copper catalyst, namely copper chloride, or extremely fine aerosol of a copper catalyst, namely copper chloride, to prepare the active contact mass.
2. Description of the Prior Art
Organohalosilanes, especially methylchlorosilanes, are well known reactive chemical intermediates used extensively in the silicone industries. The direct reaction of silicon with organic halide to produce the corresponding organohalosilanes was first disclosed by Rochow in the United States in 1945 and, at about the same time, by Muller in Germany. The direct combination of silicon and organic halides to produce organohalosilanes is unique and, hence, it is generally referred to as the "direct reaction," the "direct synthesis" or the "direct process". Rochow's process was a significant development from the prior, much more dangerous Grignard reaction for producing silanes on a large commercial scale. The direct reaction is practiced by major silicone manufacturers which produce virtually all commercial organohalosilanes in the work today. Since the early work of Rochow, the direct reaction for producing organohalosilanes has been modified and refined in many ways to improve the overall process efficiency and better utilization of raw materials. In the modern manufacture of silicones, hundreds of million pounds of organohalosilanes, methylchlorosilanes in particular, are produced annually worldwide by the direct reaction. Thus, even a small increment of improvement in the direct reaction can have a significant economic impact on the manufacturing of silicone products and, therefore, would be quite attractive to the manufacturers.
It is important to note that many organohalosilanes are produced in the direct reaction of a organic halide with silicon. For example, the direct reaction of methyl chloride and silicon produces minor amounts of tetramethylsilane, trichlorosilane, dimethylchlorosilane, silicon tetrachloride and larger amounts of methyldichlorosilane, trimethylchlorosilane, methyltrichlorosilane, dimethyldichlorosilane and methylchlorodisilanes. In modern product processes the largest volume silane manufactured is dimethyldichlorosilane which is the most useful raw material for most of the high volume commercial silicone products after it has been hydrolyzed and condensed to the siloxane form from which the term "silicones" is derived. On the other hand, the second largest volume silane produced is methyltrichlorosilane which is the least useful. The bulk of methyltrichlorosilane produced has been simply destroyed in the past when environmental protection was lax and waste disposal was inexpensive. Today this large excess methyltrichlorosilane is often sold at below manufacturing cost to make low products such as fume silica. Those skilled in the art are interested in the selectivity of the direct reaction particularly in the formation of dimethyldichlorosilane, Me.sub.2 SiCl.sub.2, in comparison with the least desirable methyltrichlorosilane, MeSiCl.sub.3. The selectivity is expressed as the ratio of methyltrichlorosilane to dimethyldichlorosilane in the crude reaction product mixture. A smaller ratio indicates less amount of the methyltrichlorosilane produced. Generally this ratio is also referred to as Me/Me.sub.2 (from the number of Me groups in the chemical formula) or T/D (from the name tri-chloro and di-chloro). For the purpose of this invention the selectivity is indicated as T/D. To the silicone manufacturers it is important to carry out the direct process to maximize the production of dimethyldichlorosilane or, conversely, to achieve a smaller value of the T/D ratio which indicates an increase in the output of the preferred dimethyldichlorosilane with a corresponding decrease of the least desirable methyltrichlorosilane.
Rochow in U.S. Pat. No. 2,380,995 issued Aug. 7, 1945 showed that the direct reaction of methyl chloride with silicon at about 300.degree. C. yields a silane mixture which was predominantly methyltrichlorosilane and dimethyldichlorosilane with a T/D ratio of 3.6. Rochow also discloses the use of a 50/50 weight ratio of silicon-copper alloy and the use of metallic catalysts other than copper, such as nickel, tin, antimony, manganese, silver and titanium. Rochow and Gilliam in U.S. Pat. No. 2,383,818 issued Aug. 28, 1945 disclose the use of contact masses prepared from silicon and an oxide of copper. Ferguson and Sellers in U.S. Pat. No. 2,443,902 issued Aug. 28, 1948 disclose a method to increase the yield of dialkyldihalosilanes from the direct reaction of alkyl halide and silicon in the presence of a finely divided cuprous catalyst, having the major constituent of the metallic copper core surrounded by a protective film of cuprous oxide on its surface. Two examples cited in the patent on the direct reaction of methyl chloride and silicon in the presence of the copper catalyst show T/D ratios of 0.18 and 0.77, respectively. Gilman in U.S. Pat. No. 2,464,033 issued Mar. 8, 1949 discloses the use of copper halides, in addition to copper metal and copper oxides, as catalysts in the direction reaction. Further, this patent discloses the use of promoters, such as zinc, zinc halide or their mixture. Improved T/D ratios ranging from 0.20 to 0.40 were demonstrated in the direct reaction of methyl chloride and silicon in the presence of the copper catalyst and zinc promoter.
Soon after Rochow's discovery, it was quickly recognized that the direct reaction is a viable commercial process to produce large quantities of the dimethyldichlorosilane and other useful methylchlorosilane monomers to make a class of completely new, siloxane polymeric materials which are generally called silicones. What followed were many new products and new uses of silicones which usages have rapidly grown to become the large, profitable business known as the silicone industry today. The direct reaction is the key production process which produces, directly or indirectly, all the starting organochlorosilanes used by the silicone industries. Since the early work of Rochow, the direct reaction has been refined in many ways to improve process efficiency and raw material utilization by improving selectivity with higher dimethyldichlorosilane yield. The teaching of the prior art on the direct reaction can be found in many publications an in textbooks, for example, "Organosilicon Compounds" by C. Eaborn, Butterworth Scientific Publications, London, 1960; "Synthesis of Organosilicon Monomers" by a. D. Petrov, B. F. Mironov, V. A. Ponomarenko and E. A. Chernyshev, Consultants Bureau, New York, 1964; and "Organohalosilanes: Precursors to Silicones" by R. J. H. Voorhoeve, Elsevier Publishing Company, New York, 1967. The teaching of the prior art concludes that copper is the most important catalyst for the direct reaction of methyl chloride and silicon to produce the methylchlorosilane monomers. Without copper, the performance of the direct reaction is so poor that it would have no commercial value by present day standard. Therefore, every silicone manufacturer who practices the direct reaction utilizes a copper catalyst in its production process. This mixture of silicon and copper catalyst used in the direct reaction is generally called the silicon/copper contact mass or, the contact mass in short. The performance of the silicon/copper contact mass in the direct reaction is also enhanced by the presence of other elements generally known as promoters. Examples are zinc, tin, antimony, cadmium, and phosphorous to name a few. Even after more than forty years since Rochow's discovery of the direct reaction, new promoters are still being discovered. Halm et al in U.S. Pat. No. 4,762,940 issued Aug. 9, 1988 disclose metal arsenides and alloys of arsenic as promoters for the direct reaction to prepare alkylhalosilanes. However, the utilization of copper as an absolutely indispensable catalyst for the direct reaction remains unchanged to the present time. As taught in the prior art, one of the most important steps to carry out an efficient direct reaction for the production of methylchlorosilane monomers is the preparation of a good contact mass from silicon and the copper catalyst.